Pulse combustor heat sources have been known and available for many years. Such systems operate on an interruptible combustion process that, once started, is self-igniting.
The combustion process of a pulse combustor starts with the mixture of a fuel and air charge in a combustion chamber. An ignition source, such as a spark ignitor, is employed to cause the mixture to burn thereby rapidly increasing the pressure and temperature within the combustion chamber. The increased pressure forces the combustion products to leave the combustion chamber through a tailpipe where they perform the desired work.
The momentum of the gases leaving the combustion chamber lowers the pressure in the combustion chamber to the point where a new charge of combustion air and fuel can be admitted to the combustion chamber. At the same time the reduced combustion chamber pressure causes a small portion of the exhaust gas to return to the combustion chamber where it becomes the ignition source for a subsequent combustion event. A rapid increase in pressure occurs which again forces combustion chamber products to leave the combustion chamber thereby repeating the cycle. The cycle then repeats again and again until the pulse combustor achieves a desired operating temperature level and a steady-state natural operating frequency. As indicated, the process is self-igniting, which eliminates the need for an ignition source for each pulse.
Most pulse combustors are unstable when they are "cold" during the short period necessary for coming up to operating temperature. This is usually caused by a lack of radiation energy from the walls of the combustion chamber, which can contribute to the ignition process, and by variations in the speed of sound due to temperature. This instability is caused by the combustor attempting to operate at several frequencies while it seeks the natural frequency during the initial phase of the combustion process.
Prior art pulse combustors use direct spark ignitors (typically a conventional spark plug), which are located inside the combustion chamber, to initiate the combustion process and sustain it until the operating temperature is achieved or until the process is self-sustaining. Such spark ignitors consist of two electrodes which have a preset distance between them. To initiate ignition, a current is sent to an ignition transformer where the voltage is increased from 110 volts to a range from 3,000 to 10,000 volts. This high voltage causes a spark to develop between the two electrodes in the combustion chamber, which in turn starts the combustion process.
It has been found that the heat from the combustion process, as well as the heat generated by the spark between the two electrodes, will cause an excessive temperature build-up on the electrodes and cause them to melt or oxidize. This shortens their life and results in increased maintenance cost for the pulse combustor system.